1. Field of the Invention
The present invention relates to a switching regulator and a control circuit and method used therein, and more particularly, to a buck-boost switching regulator and a control circuit and method used therein, which perform voltage regulation using synchronous rectification and can provide effective overcurrent protection during the voltage regulation.
2. Discussion of the Background
Buck-boost switching regulators are used in power supplies of various electronic devices to obtain a desired voltage from a wide range of input voltage.
For example, compact portable electronic devices such as mobile phones, which have become popular in recent years, use buck-boost switching regulators to extend acceptable voltage range of a power supply used therewith. Such electronic devices commonly use secondary batteries, and attempts have been made to achieve longer operating times with smaller secondary batteries, for example, by enhancing battery performance and developing products with high energy efficiency. Using a buck-boost switching regulator in an electronic device with a secondary battery enables the device to normally operate with a wide range of input voltage supplied from the battery, thus enhancing energy efficiency. Further, the buck-boost switching regulator makes the electronic device compatible with various power supplies such as alternating current (AC) adaptors as well as batteries.
Buck-boost switching regulators are also used in power supplies with a high output voltage dynamic range, e.g., a power supply used in an amplifier for signal transmission. As these regulators can convert an input voltage to an output voltage that is either higher or lower than the input voltage, the use of a buck-boost switching regulator for such purpose significantly improves power efficiency.
For example, a buck-boost switching regulator used in a mobile phone amplifier serves to provide an appropriate supply voltage to the amplifier in accordance with the distance between the phone and a base station, that is, reducing the supply voltage when the distance is relatively short and increasing the supply voltage when the distance is relatively large.
In addition, switching regulators using inductors are widely used in battery-powered electronics for simplifying circuit design and achieving high energy efficiency.
Typically, an inductor-based buck-boost switching regulator includes an overcurrent protection circuit for preventing an overcurrent condition, which occurs when the output voltage rapidly rises, e.g., upon power-on, and can result in an excessively large current flowing to load circuit components connected thereto.
Referring to FIG. 1, a circuit diagram illustrating an example of a conventional buck-boost switching regulator 100 having an overcurrent protection circuit is described.
The buck-boost switching regulator 100 includes first through fourth transistors M101 through M104, an inductor L, first and second resistors R131 and R132, a current control circuit 131, a switching control circuit 132, an overcurrent detection circuit 133, first and second voltage sensors 134 and 135, and a driver 136.
The buck-boost switching regulator 100 receives a voltage input from a terminal IN, performs voltage regulation, and outputs a regulated voltage to a terminal OUT. The output voltage is higher than the input voltage in a boost (step-up) mode and lower than the input voltage in a buck (step-down) mode.
In the buck mode, the first transistor M101 turns on/off to control the output voltage while the second transistor M102 turns on/off to perform synchronous rectification. Similarly, in the boost mode, the third transistor M103 turns on/off to control the output voltage while the second transistor M104 turns on/off to perform synchronous rectification.
The first voltage sensor 134 detects voltage across the first resistor R131, and the second voltage sensor 135 detects voltage across the second resistor R132. The overcurrent detection circuit 133 receives signals from the voltage sensors 134 and 135, respectively, and outputs a signal indicating an overcurrent condition when a current level detected from the received signals is higher than a specified reference level. Upon overcurrent detection, the overcurrent detection circuit 133 outputs a signal to the switching control circuit 132, so that the switching control circuit 132 controls the driver 136 to turn off the first through fourth transistors M101 through M104.
A drawback of such overcurrent protection function is that it may result in a failure to raise the output voltage to a sufficient level in rapidly boosting the output voltage, e.g., upon power-on, which causes considerable inconvenience when the switching regulator is used in the mobile phone amplifier application, where output voltage level rapidly changes from low to high or vise versa at frequent intervals.
Therefore, a conventional buck-boost switching regulator is designed so that the overcurrent protection function does not work during power-on, or that the reference level for overcurrent detection is set to be unreasonably high. Neither alternative is desirable.